The present invention relates to a method and a device for controlling brushless direct-current electric motors, commonly known by the acronym BLDC, which allow the advantageous application of said motors to the movement of the weft winding arm in weft feeders for weaving looms.
It is known that weft feeders are devices which comprise a fixed drum on which a weft winding arm winds, as in a fishing reel, a plurality of turns of thread which constitutes a weft reserve. Said turns unwind from the drum of the feeder, when requested by the loom, at each weft insertion, and the weft winding arm, under the control of a supervisor microprocessor, winds new turns in order to restore the weft reserve.
The motor that is currently most widely used for moving the weft winding arm is the three-phase asynchronous type. This choice is essentially dictated by the inherent characteristics of these motors and mainly by their low installation and maintenance costs, afforded by their simple and sturdy structure and by their complete lack of elements in mutual sliding contact.
Moreover, the evolution of semiconductor technology has made available control microprocessors which integrate peripherals capable of directly generating the waveforms of the control signals for the inverter that actuates said motors, where the term xe2x80x9cinverterxe2x80x9d designates the driving device capable of generating a system of multiple-phase sinusoidal voltages whose amplitude and frequency can be varied at will.
However, although said three-phase asynchronous motors yield satisfactory efficiency in terms of performance/cost ratio, they have some drawbacks which limit said performance, especially when applied to the movement of said weft winding arm of weft feeders.
The greatest of these drawbacks is that it is impossible to achieve effective and simple control of the torque delivered by the motor. For this purpose it is in fact necessary to resort to sophisticated control systems of the vector type, which however, due to the large number of sensors required (at least two for phase current control and one for detecting the rotation rate) and to the high computing power requirement, are not adapted for low-cost installation on said weft feeders. Accordingly, such expensive and complicated control systems of the vector type are avoided, by typically assuming, for motor speed adjustment, an open-loop system, leaving the synchronization speed set by the inverter to be chased by said motor.
In this manner, however, it is not possible to obtain high-level dynamic performance from the motor, and this is a severe drawback if the weft winding arm of said feeder is required to provide high accelerations and decelerations, as increasingly often occurs due to the continuous increase in weaving speed.
Moreover, with the open-loop adjustment system the current absorbed by the motor is often significantly higher than actually required, and therefore the excess absorbed power is dissipated as heat, causing dangerous overheating of the motor and of the electronic components of the power section of the inverter.
The aim of the present invention is to eliminate these severe drawbacks by replacing, for the movement of the weft winding arm in said weft feeders, the three-phase asynchronous motor with a motor of the above specified brushless direct-current type and by providing a method and a device for controlling said brushless motor which are particularly adapted to meet the operating requirements of modem weft feeders.
The advantages provided by the use of a brushless motor instead of the three-phase asynchronous motor substantially consist in the possibility to directly and easily control the torque by way of the corresponding control of the current that circulates in the switched stator phases; in the improved power/volume ratio, with a consequent and corresponding reduction, for an equal delivered power, in the dimensions of the motor and, as a whole, of the feeder; in the reduced inertia of the rotor, allowing better accelerations; and in the simplification of the stator windings by means of the adoption of diametrical turns.
However, conventional devices currently used for driving a brushless motor typically have a supervisor microprocessor, an angular velocity sensor and a set of three position sensors designed to encode the angular position of the rotor in steps of 60 electrical degrees. This information is in fact essential in order to allow the supervisor microprocessor to switch the correct stator phases, i.e., the phase currents that generate a stator flux in quadrature with the rotor flux generated by the permanent magnets.
The presence of said angular velocity and position sensors, however, significantly complicates the structure of the driving device and offsets most of the above-listed advantages which are typical of brushless motors, on the one hand by significantly increasing the installation costs of said motors and their overall dimensions and on the other hand by equally significantly reducing the reliability of the motor/control system.
The aim of the present invention is to provide a method and a device for controlling brushless motors which, by eliminating the drawbacks of conventional control systems, make it advantageous to apply said motors to the movement of the weft winding arm, maintaining its inherent characteristics, which are particularly favorable for this application, with all the consequent advantages.
Within the scope of this aim, an object of the present invention is to provide an adjustment method and device which are simple, inexpensive and highly efficient and reliable.
According to the present invention, these and other objects which will become better apparent hereinafter are achieved with a method and a device for controlling electric motors of the brushless type which have the specific characteristics stated in the appended claims.
Substantially, the invention is based on the concept of eliminating, in the control system, the angular velocity sensor and the set of three sensors that encode the angular position of the rotor, and of determining said velocity and position by deducing them from the sampled reading of the phase voltages of the motor, providing the supervisor microprocessor with corresponding information useful for stator phase switching. Moreover, according to the invention, the current of each phase is measured by means of a shunt resistor which is inserted on the negative conductor of the three-phase power supply bridge, since only one current at a time circulates in brushless motors.